Ion guides are used in spectrometers and other devices to transport ions and for other purposes. Ions are provided using an ion source. For most atmospheric pressure ion sources, ions pass through an aperture or skimmer prior to entering the ion guide at an ion entry end. A radio frequency signal may be applied to the ion guide to provide radial focusing of ions within the ion guide. As a result, the transport efficiency through an ion guide can be very high.
Some ion sources, including matrix assisted laser desorption/ionization (MALDI), surface enhanced laser desorption/ionization (SELDI) and other ion sources are capable of generating ions in lower pressure regions. When such an ion source is used with an ion guide, the ion source may be positioned adjacent to the ion entry end of the multipole such that the ion generation region and the multipole are maintained at the same pressure. Some of the ions generated from the ion source enter the ion guide. When there is little or no pressure differential between the source and the ion guide, ions are typically propelled along the length of the ion guide by space charge repulsion between the ions that have the same polarity. As new ions are generated during a particular experiment and enter the ion guide, previously generated ions are propelled along the length of the ion guide by space charge repulsion. While space charge effects will propel ions through an ion guide, they can lead to a number of undesirable effects. For instance, the extent of the axial force on an ion depends on both the number and proximity of other ions of the same polarity. As a result, the transport of the ions through the ion guide is inconsistent and slow when space charge is the dominant driving force. For MALDI quantitation experiments, where samples can be ablated to depletion on the target, ion liberation rates from samples are initially high and then drop off to zero over the course of an experiment. Therefore, the space charge force is strong initially, and subsequently drops off such that ions generated near the end of the experiments are more weakly propelled through the ion guide. This can lead to broad and variable peak shapes, unsuitable for high throughput quantitation. In addition, since space charge forces are essentially non-directional, ion losses are expected to be greater when they comprise the most significant driving force for ion motion in the axial direction.
It is desirable to provide an ion guide with a more efficient ion transport mechanism than previous devices to more efficiently and reproducibly transport ions along the length of an ion guide.